For certain valvular lesions due to various causes, it is sometimes necessary to implant artificial heart valves, the mechanical structure of which allows for a mitral or aortic valvular unit to be replaced.
Considering the functions an artificial heart valve must fulfill, insofar as possible, it is important the latter meets several criteria:
The valve must ensure its function for a period beyond the life expectancy of the patient regardless of his/her age at the time when the valve was implanted, Therefore, the characteristics of the valve must be determined accordingly.
The valve must provide a central laminar flow without upstream or downstream turbulences in order to meet hemodynamic performances so as not to create significant transvalvular gradients through the implant, This is particularly the case for valves with a small diameter.
Considering the presence of stasis areas resulting from the principle of the mechanism of the artificial valves, fibrocruoric clots may form at the level of this mechanism thereby being likely to break away from and migrate into the systemic circulation. Besides, thrombosis responsible for blockage of the valve mechanism may result from this.
In order to reduce this phenomenon of thrombogenicity, an anticoagulant treatment administered by oral route is usually called for, This treatment is aimed at limiting the formation of clots at the valve mechanism.
To date, several types of valves have been offered with different technological designs, determined in an attempt to achieve the aforementioned aims as much as possible.
For example, valves, the mechanism of which mainly includes a ball freely moving in a metal cage with bows integral to a metal ring, are known. A ball in the blood flow, can be responsible for a mainly lateral flow, meaning the hemodynamic performances are not satisfactory.
In order to improve these hemodynamic performances, a valve with a rocking disk creating a lateral flow and a main area and secondary area, was offered. However, it was noticed that this secondary area is responsible for stasis, consequently leading to thrombosis phenomena even in the case of anticoagulant treatment.
In an attempt to overcome these disadvantages and solve the problems relating to the hemodynamics and thrombogenicity, artificial heart valves with two wings hinged inside a ring, are proposed. The valve known under the name of the "ST JUDE MEDICAL" valve can be quoted as an example, with two flat wings fitted with hinges in the ring of the prosthesis and allowing opening of the said wings at an angle in the region of 85 degrees. Consequently a virtually central and laminar flow results. Furthermore, with this type of valve, the plane of the wings in the open position, exceeds the plane of the ring very slighty, thus avoiding, (particularly in the mitral position), interfering with the opening and closing movement of the wings by sub-jacent anatomical structures.
However, it appears that when the wings are in the open position, the valve does not have three equivalent areas. The two lateral areas each represent virtually 40 percent, whereas the central area only represents 20 percent. Therefore, it appears that the central area is less used than the lateral areas so that stasis phenomena at this level with insufficient blood flushing, can generate a phenomenon of thrombosis requiring anti-coagulant treatment.
Other types of valves with curved wings are also known. As opposed to those described above, these have three equivalent areas when the wings are in the open position. Clinical tests have shown that the adoption of three equivalent areas, significant limits valvular thrombosis phenomena. On the other hand, if one could consider that the problem of thrombogenicity was overcome, the same cannot be applied to the problem of hemodynamics. In fact, the opening of the wings is limited to approximately 76 degrees, creating oblique flow due to this in the lateral areas. Another disadvantage also appears concerning the technological design in that when the valve is in the open position, the wings are well outside the ring which can lead to blocking incidents of the said wings by sub-jacent ventricular structures, particularly in the mitral position.
This state of the art can be illustrated by the teaching of the Patent EP 0338179. Besides the fact that when the wings are in the open position, they are well outside the plane of the ring, other disadvantages and problems arise considering the structure of the valve.
A first disadvantage arises at the hinge of the wings in the ring which does not limit (between the ends of the wings) a totally smooth surface but profiled recesses likely to interfere with the central laminar flow.
Another disadvantage also appears on closure of the wings which are applied against the ring, which gives rise to a repetitive noise for the patient and those around him/her.
Finally, it is highlighted, considering the type of material making up the wings, the latter are relatively thick meaning they do not have enough elasticity to absorb the kinetic energy stored by the speed of the blood flow.